1. Field of the Invention
The present invention generally relates to a clamping apparatus, and more particularly, is directed to a signal level clamping apparatus for a CTDM video signal used for a time-base correction apparatus which is suitable for application to a time-base correction apparatus of a Betacam video tape recorder (VTR).
2. Description of the Prior Art
According to the conventional Betacam VTR, each of a luminance signal and a time-compressed color difference signal is pre-emphasized and is separately subjected to frequency modulation (FM), then both the frequency-modulated luminance signal and the frequency-modulated color difference signal are recorded on a magnetic tape by a rotary magnetic head to form slant tracks on the tape. During reproduction, the frequency-modulated luminance signal and the frequency-modulated color difference signal are reproduced from the magnetic tape by the rotary magnetic head and then subjected to frequency demodulation to thereby obtain the luminance signal and the time-compressed color difference signals which are then de-emphasized. The de-emphasized luminance signal and time-compressed color difference signals are supplied to a time-base correction apparatus for time-correcting these signals.
In the time-base correction apparatus, the de-emphasized time-compressed color difference signal is supplied to a time-base correction unit through a clamping circuit. The de-emphasized time-compressed color difference signal as shown in FIG. 1A includes, between adjacent two horizontal synchronizing (sync.) signals, two kinds of time-compressed color difference line signals, that is, a red color difference line signal (R - Y) and a blue color difference line signal (B - Y); sequentially arranged with a predetermined time interval or space therebetween. A reproduced horizontal sync. signal is extracted from the reproduced color difference signal by a sync. separating circuit, then a clamping pulse shown in FIG. 1B and a write clock signal are generated in synchronism with the reproduced horizontal sync. signal. The clamping pulse and the write clock signal are applied to the clamping circuit and the time-base correction unit, respectively, thereby clamping in response to the clamping pulse a pedestal portion within a space between each of the horizontal sync. signals and the color difference line signal contiguous thereto of the reproduced color difference signal, as shown in FIG. 1.
However, in the conventional clamping apparatus for a time-base correction apparatus, while there is no problem in the normal playback mode of the VTR, constants in a de-emphasizing circuit in the variable speed playback mode become different from those of the pre-emphasizing circuit. Thus, an overshoot portion or an undershoot portion appears at a trailing edge portion of the horizontal sync. signal as shown in phantom in FIG. 1A. Accordingly, the prior-art clamping apparatus has the disadvantage that the clamping operation can not be performed accurately when clamping a pedestal portion within a space between each of the horizontal sync. signals and the color difference line signal contiguous thereto of the reproduced color difference signal.